The present invention relates to electrical relays and more specifically to a modular trip free relay suitable for direct connection to a printed circuit board.
Overload relays are electrical switches typically employed in industry to protect equipment from excessive current conditions that in turn cause overheating, performance degradation and eventually equipment malfunction or destruction. For instance, a three phase induction motor is often linked to a power source through a relay commonly referred to as a contactor. A typical contactor includes a separate power path for each of the three motor phases. Contactor motion is typically provided magnetically as the result of power flow through a coil where the current though the coil is controlled by a control switch.
In many cases an overload relay is connected in series with the control switch for the contactor coil. When overload conditions occur, the overload relay opens thereby cutting off power to the contactor coil. When power to the coil is discontinued, the coil opens and power to the equipment is cut off.
Many overload relays have been designed such that, once tripped, the relay remains open to prevent current flow to the contactor until the relay is manually reset by a system operator. A common resetting device is a reset push button selectable by an operator to reset the relay thereby allowing current to flow to and to close the contactor coil which in turn provides current to the linked equipment.
For some applications industry standards require that re-settable relays be openable when overload conditions occur even if the reset button is continually inadvertently or manually held down. These overload relays that are openable even while the reset button is pressed are generally referred to as “trip free” relays and that term will be used hereinafter to refer to such configurations.
An exemplary trip free relay configuration includes a bi-stable armature that is operably linked to contacts to open a first set of contacts and close a second set of contacts when in a set position and to close and open the first and second sets when in a tripped position. Here, to reset the armature and hence the contacts after the relay trips, a button and a rigid linking member are provided where the rigid linking member is spring mounted to the armature and extends toward and contacts the button when the armature is in the tripped position. When the button is pressed, button force is transferred through the linking member to the armature thereby causing the armature to rotate toward the set position. The linking member is designed so that, as the armature approaches the set position, the linking member decouples from the button. If an over current condition occurs after the linking member decouples from the button and while the button is pressed, the relay can assume the tripped position again.
There are other advantageous features that may be included in a relay. For example, for test purposes, it is advantageous to provide a relay configuration where the relay can be manually tripped (i.e., a “manual trip” feature). As another example, it is sometimes advantageous to provide a relay where at least one of the normally closed relay contacts can be opened for a short period to momentarily interrupt power to linked equipment (i.e., an “open circuit” feature). As one other example, sometimes it is advantageous to provide a relay that can be automatically reset when overload conditions cease to exist (i.e., an “automatic reset” feature). A relay configuration including all of the features described above will be referred to hereinafter as a “fully featured” relay.
In addition to the mechanical components described above, a fully featured relay assembly also typically includes a printed circuit board (PCB) including control circuitry for tripping and automatically resetting the relay, current sensors and various types of terminals for linking to power lines, the contactor and perhaps indicating lights.
Past known mechanical trip free relay configurations have been designed to include a housing generally forming a single housing compartment or cavity including features for mounting all of the required trip free relay components. For instance, an exemplary known trip free housing assembly includes structure for mounting a trip free sub-assembly, a manual reset sub-assembly, an open circuit sub-assembly, the PCB, the current sensors and the connection terminals. Here it has generally been believed that a reduced parts count when a single housing was employed would result in reduced manufacturing costs.
Unfortunately, while the single housing approach to trip free relay design generally reduces parts count, expected cost savings from parts count reduction has never been realized. The disconnect between parts count and cost is in great part due to increased manufacturing difficulties attributable to the single housing designs. To this end, while the size of a typical single housing assembly is generally increased to accommodate all of the required trip free relay components, tolerances between housing structure for mounting components have not been commensurately increased. Here, for instance, trip free sub-assembly tolerances generally have to remain within a relatively small range in order for a trip free sub-assembly to work properly. As well known in the manufacturing arts, where dimensions (e.g., housing dimensions) are increased and tolerances (e.g., trip free sub-assembly tolerances) remain the same, more precise and hence more costly manufacturing techniques have to be adopted to meet tolerance limits. Thus, more complex manufacturing techniques required to manufacture single housing trip free relays have generally increased relay costs more than the reduction in parts count has reduced costs.